Fruit removal in soybean induces the formation of an insoluble form of ribulose-1,5-bisphosphate carboxylase/oxygenase in leaf extracts*

In some soybean (Glycine max (L.) Merr.) cultivars, fruit removal does not delay the apparent loss of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activity and abundance or the decline in photosynthesis. Analysis of leaf extracts from defruited plants indicated a time-dependent increase in both Rubisco activity and abundance in a 30000 · g pellet fraction in cultivars which had been reported to lose all Rubisco protein from the supernatant fraction. Attempts to solubilize the pelleted Rubisco by increasing the buffer volume/tissue ratio or by adding alkylphenoxypolyethoxyethanol (Triton X-100), ethylenediaminetetraacetic acid (EDTA), or NaCl were unsuccessful. However, treatment of the pellets with denaturants such as 8 M urea or 5% (w/v) sodium dodecyl sulfate (SDS) did release Rubisco from the pellet. Redistribution of protein to the pellet fraction appeared to be specific for Rubisco since the amount of ribulose-5-phosphate kinase (EC 2.7.1.19) found in the pellet fraction of leaf extracts of control and defruited plants was small and constant over time. The loss of soluble Rubisco, and the concomitant increase in insoluble Rubisco, in response to fruit removal varied with genotype and was reproducible in both field and greenhouse environments. In addition, the effect was influenced by node position and light; lower and-or shaded leaves exhibited less Rubisco in the pellet fraction than leaves from the top of the plant that was fully exposed to sunlight. When isolated by sucrose-density-gradient centrifugation, the insoluble Rubisco was found to co-purify with a 30-kDa (kilodalton) polypeptide. These results indicate that alteration of the source/sink ratio by removing fruits results in the formation of an insoluble form of Rubisco in leaf extracts of soybean. Whether or not Rubisco exists as an insoluble complex with the 30-kDa polypeptide in intact leaves of defruited plants remains to be determined.Abbreviations kDa kilodalton - PGA kinase 3-phosphoglyceric acid kinase (EC 2.7.2.3) - Rubisco ribulose-1,5-bisphosphate car-boxylase/oxygenase (EC 4.1.1.39) - Ru5P kinase ribulose-5-phosphate kinase (EC 2.7.1.19) - SDS-PAGE sodium dodecyl sulfatepolyacrylamide gel electrophoresis     

The absence of Rubisco activase activity in total wheat leaf extracts is recovered in the purified protein

When desalted extracts of soluble protein from dark-adaptedwheat leaves were assayed for ribulose-1, 5-bisphosphate carboxylase/oxygenase(Rubisco) activase activity in the presence of 1 mM ATP andan ATP-regenerating system, very little ATP-dependent activationof RuBP-inactivated Rubisco was found. In extracts from light-adaptedleaves a very similar pattern of Rubisco activation was observedexcept that the overall level of Rubisco activity was much lowerthan in the extracts from dark-adapted leaves. These featureswere apparent both at low (120µg per ml) and high (640µg per ml) protein concentrations. We were unable to demonstrateRubisco activase activity in crude leaf extracts. Consequently,in order to establish that Rubisco activase was present in wheatleaf extracts the wheat leaf protein was purified to homogeneity.The identity of the protein was confirmed with antibodies tothe spinach enzyme, ATPase activity and activase-mediated releaseof the inhibitor, carboxyara-binitol-1-phosphate (CA1P) fromthe tertiary Rubisco complex. The pure wheat Rubisco activaserelieved the CA1P-induced inhibition of Rubisco activity. Rubiscoactivase had no significant effect on the affinity of wheatRubisco for the substrate, ribulose-1, 5-bisphosphate (RuBP). Key words: Rubisco activase, Rubisco, regulation     

Degradation of the Large Subunit of Ribulose-1, 5-Bisphosphate Carboxylase/Oxygenase in Wheat Leaves

The degradation of the large subunit (LSU) of ribulose- 1, 5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) in wheat (Triticum aestivum L. cv. Yangmai 158) leaves was investigated. A 50 kDa fragment, a portion of the LSU of Rubisco, was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with antibody against tobacco Rubisco in crude enzyme extract of young wheat leaves. The appearance of the 50 kDa fragment was most obvious at 30-35 ℃ and pH 5.5. The LSU and its 50 kDa fragment both existed when the crude enzyme extract was incubated for 60 min. The amount of LSU decreased with incubation time from 0 to 3 h in crude enzyme extract. However, the 50 kDa fragment could not be found any pH from 4.5 to 8.5 in chloroplast lysates of young wheat leaves. In addition,through treatment with various inhibitors, reactions were inhibited by cysteine proteinase inhibitor E-64 or leupeptin.     

Changes in ribulosebisphosphate carboxylase/oxygenase and ribulose 5-phosphate kinase abundances and photosynthetic capacity during leaf senescence

The abundances of ribulose-1,5-bisphosphate carboxylate/oxygenase (Rubisco) and ribulose-5-phosphate (Ru5P) kinase in field-grown soybean (Glycine max L. Merr.) leaves were quantified by a Western blot technique and related to changes in chlorophyll and photosynthetic capacity during senescence. Even though the leaf content of Rubisco was approximately 80-fold greater than that of Ru5P kinase, the decline in the levels of these two Calvin cycle enzymes occurred in parallel during the senescence of the leaves. Moreover, the decrease in the content of Rubisco was accompanied by parallel decreases of both the large and small subunits of this enzyme but not by an accumulation of altered large or small subunit isoforms. With increasing senescence, decreases in abundances of Rubisco, Ru5P kinase and chlorophyll were closely correlated with the decline in photosynthetic capacity; thus, the specific photosynthetic capacity when expressed per abundance of any of these parameters was rather constant despite an 8-fold decrease in photosynthetic capacity. These results suggest that during senescence of soybean leaves the chloroplast is subject to autolysis by mechanisms causing an approximately 80-fold greater rate of loss of Rubisco than Ru5P kinase.Jointly supported by the United States Department of Agricultural Research Service and the Kentucky Agricultural Experiment Station, Lexington (paper No. 88 3 286).Mention of a commercial product does not constitute endorsement by the United States Department of Agriculture.     

Dithiothreitol triggers photooxidative stress and fragmentation of the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase in intact pea chloroplasts

In intact chloroplasts isolated from mature pea leaves (Pisum sativum L.), the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was rapidly fragmented into several products upon illumination in the presence of 1 mM dithiothreitol (DTT). Very similar effects on LSU stability could be observed when illuminated chloroplasts were poisoned with cyanide which, like DTT, inhibits important plastid antioxidant enzymes, or when a light-dependent hydroxyl radical-producing system was added to the incubation medium. Moreover, DTT-stimulated light degradation of LSU was markedly delayed in the presence of scavengers of active oxygen species (AOS). It is therefore suggested that light degradation of LSU in the presence of DTT is mainly due to inhibition of the chloroplast antioxidant defense system and the subsequent accumulation of AOS in intact organelles. When chloroplasts were isolated from nonsenescent or senescent leaves, LSU remained very stable upon incubation without DTT, indicating that the antioxidant system was still functional in the isolated chloroplasts during leaf ageing. Our data support the notion that AOS might be important for the degradation of Rubisco in vivo under oxidative stress.     

Solubilization of ribulose-1,5-bisphosphate carboxylase from the membrane fraction of pea leaves

The solubilization of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from the membrane fraction was studied in whole leaf extracts and chloroplasts from pea. The amount of membrane-bound Rubisco was dependent on the pH of the chloroplastic lysate buffer. Maximum binding was found at pH 8.0, with about 8% of total leaf Rubisco being bound. The binding of Rubisco to the membranes was strong, and it was not released by repeated washing with hypotonic buffer or by changing ionic strength. Detergents such as Triton X-100, Tween 20, deoxycholate and dodecylsulfate were effective in solubilizing the membrane-bound Rubisco. Triton X-100 was most effective in the range of 0.04% to 0.2% and it solubilized Rubisco from the membrane without any decrease in enzyme activity.Abbreviations BSA bovine serum albumin - CABP carboxyarabinitol-1,5-bisphosphate - DTT dithiothreitol - LDS lithium dodecylsulfate - LHC light-harvesting chlorophyll protein complex - RuBP ribulose-1,5-bisphosphate - Rubisco RuBP carboxylase/oxygenase - SDS sodium dodecylsulfate - SDS-PAGE SDS-polyacrylamide gel electrophoresis     

A point mutation in the N-terminus of ribulose-1,5-bisphosphate carboxylase affects ribulose-1,5-bisphosphate binding

Mutagenesis in vitro of the gene encoding the large subunit of ribulose-1,5-bisphosphate carboxylase/ oxygenase (EC 4.1.1.39) from Anacystis nidulans was used to generate novel enzymes. Two conserved residues, threonine 4 and lysine 11 in the N-terminus were changed. The substitution of threonine 4 with serine or valine had little effect on the kinetic parameters. The substitution of lysine 11 with leucine, which is non-polar, increased the K _m for ribulose-1,5-bisphosphate from 82 to 190 M but its replacement with glutamine, which has polar properties, had no appreciable effect.Abbreviations Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - LSU large sub-unit of Rubisco - SSU small subunit of Rubisco We thank Dr. S. Gutteridge (DuPont, Wilmington, USA) for structural information and for his comments on the results described. The technical assistance of Mr. A. Cowland and Mr. I. Major was invaluable.     

Suboptimal nitrogen status sensitizes the photosynthetic apparatus in willow leaves to long term but not short term water stress

The response to drought was compared for willow plants of optimal leaf nitrogen content (100 N) and those of 86% of this content (86 N). Gas exchange measurements revealed that the carboxylation efficiency (CE) of photosynthesis was more sensitive to drought than the photosynthetic capacity in both N regimes. Since the leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was found to be much more resistant it is suggested that a decreased specific activity of Rubisco underlies the decreased CE. Although the rate of water consumption was the same for 86 N and 100 N plants the photosynthetic apparatus responded much more rapidly in the 86 N leaves. This increased sensitivity of 86 N leaves was not due to accelerated senescence as judged by comparison with parallel plants subjected to discontinued fertilization; the two categories of treatments resulted in the same loss of leaf nitrogen and Rubisco but drought induced a much more rapid photosynthetic depression. In contrast to the drought situation, 86 N and 100 N plants behaved similarly when compared under short term water stress. First, when single attached leaves were exposed to a sudden drop in air humidity the capacity of CO₂ uptake in both N regimes decreased about 20% over 10 min while the leaf water potential remained high. Second, in freely transpiring leaf discs cut from 86 N and 100 N leaves the same relationship between capacity of O₂ evolution and extent of dehydration was observed. The possible mechanisms underlying the increased susceptibility of 86 N leaves to drought is discussed; the water status of the roots not the leaves is suggested to be the determining factor.Abbreviations CE carboxylation efficiency - 100 N optimal nitrogen regime - 86 N suboptimal nitrogen regime with 86% of the optimal leaf nitrogen content, Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Photosynthetic properties of two different soybean suspension cultures

The photosynthetic properties of two commonly used suspension cultured lines, embryogenic and photoautotrophic (PA, SB-1 line) cells of soybean [Glycine max (L.) Merr.] were characterized. We found that compared to the dark green PA cells, the light green embryogenic cells contained fewer and smaller plastids with less-developed thylakoid membranes. The embryogenic cells also contained much lower contents of both chlorophyll and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) protein, an undetectable level of Rubisco small subunit protein, and a very low rate of photosynthesis. While the DNA contents of the nuclear genomes were similar in these two types of cultured cells, the embryogenic cells possessed a markedly lower content of plastid DNA. The 18-year-old PA suspension culture, SB-1, continues to evolve with higher Rubisco and plastid DNA contents than leaves, and with small decreases in nuclear DNA content that appears to mimic changes in chromosome numbers. These findings may prove useful in the application of plastid transformation, particularly when non-leaf or non-green tissues must be used as targets for transformation and plant regeneration.     

Amino-acid-transport mutant of Nicotiana tabacum L.

The structure of spinach ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) has been investigated by tilted-view electron microscopy of negatively stained monolayer crystals and image processing. The structure determined consists of a cylinder of octagonal cross-section with a large central hole. Based on this and other available evidence a model for the arrangement of the large and small subunits is suggested with the eight small subunits arranged equatorially around the core of eight large subunits.Abbreviations LS large subunit - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - SS small subunit     

The DNA-binding protease,CND41, and the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase in senescent leaves of tobacco

Plastids bear their own genome, organized into DNA–protein complexes (nucleoids). Recently, we identified a DNA-binding protease (CND41) in the chloroplast nucleoids of cultured tobacco (Nicotiana tabacum L.) cells. In this study, we examine the biochemical function of this novel DNA-binding protease, particularly in senescent leaves, because antisense tobacco with a reduced amount of CND41 showed retarded senescence. Nitrogen-depletion experiments clearly showed that CND41 antisense tobacco maintained green leaves and constant protein levels, especially ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), throughout the whole plant, whereas wild-type tobacco showed marked senescence and the reduction of protein levels in the lower leaves. In vitro analyses confirmed that CND41 showed proteolytic activity at physiological pH when denatured Rubisco was used as the substrate. These results suggest that CND41 is involved in Rubisco degradation and the translocation of nitrogen during senescence. The possible regulation of protease activity of CND41 through DNA-binding is discussed.Abbreviations CABP 2-Carboxyarabinitol-1,5-bisphosphate - CBB Coomassie Brilliant Blue - GS Glutamine synthetase - OEC33 The extrinsic 33-kDa protein in the oxygen-evolving complex - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase     

Changes in the synthesis of rubisco in rice leaves in relation to senescence and N influx

BACKGROUND AND AIMS: The amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) synthesized in a leaf is closely correlated with N influx into the leaf throughout its lifetime. Rubisco synthesis and N influx are most active in the young leaf during expansion, but are very limited in the senescent leaf. However, it is not established whether Rubisco synthesis can be observed if N influx is increased, even in a very senescent leaf. This study first investigated changes in the relationships between rbcS and rbcL mRNA contents and Rubisco synthesis per unit of leaf mass with leaf senescence. Next, leaves were removed during late senescence, to examine whether Rubisco synthesis is re-stimulated in very senescent leaves by an increase in N influx. METHODS: Different N concentrations (1 and 4 mm) were supplied to Oryza sativa plants at the early (full expansion), middle and late stages (respectively 8 and 16 d after full expansion) of senescence of the eighth leaf. To enhance N influx into the eighth leaf 16 d after full expansion, all leaf blades on the main stem, except for the eighth leaf, and all tillers were removed and plants received 4 mm N (removal treatment). KEY RESULTS: Rubisco synthesis, rbcS and rbcL mRNAs and the translational efficiencies of rbcS and rbcL mRNAs decreased with leaf senescence irrespective of N treatments. However, in the removal treatment at the late stage, they increased more strongly with an increase in N influx than in intact plants. CONCLUSIONS: Although Rubisco synthesis and rbcS and rbcL mRNAs decrease with leaf senescence, leaves at the late stage of senescence have the potential actively to synthesize Rubisco with an increase in N influx.     

Influence of nitrogen supply and sink strength on changes in leaf nitrogen compounds during senescence in two wheat cultivars

Changes in various nitrogen compounds during senescence of the fourth leaf were studied in two cultivars of spring wheat (Triticum aestivum L.). One of the cultivars (Yecora) was supplied with two N levels; the other (Tauro) was grown with the high N level and pruned above the fourth leaf, whereas the control was left intact. In both cultivars grown with high N supply, net nitrogen export from the fourth leaf did not occur until 35 days after sowing (DAS). Loss of leaf soluble proteins started earlier than that of chlorophylis, and coincided initially with an increase in insoluble protein. In N deficient plants the level of total N, soluble protein, and the activity of nitrate reductase (NRA. EC 1.6.6.1) started to decrease about 5 days earlier, and along with chlorophyll, continued to decrease at a faster rate, than in high N plants. Also, with low N supply, the large subunit (LSU, 58 kDa) of ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) decreased in greater proportion than other soluble proteins, while with high N supply the decrease in Rubisco LSU was similar to that of other soluble proteins. Nitrogen deficiency caused a greater decrease in soluble proteins than in insoluble proteins, and NRA relative to soluble proteins. The faster senescing Tauro cultivar had lower levels of most parameters, especially NRA, soluble protein and, after 35 DAS. Rubisco LSU as a proportion of soluble protein. The decrease in sink strength due to shoot pruning did generally not affect the level of the various nitrogenous compounds until 35 DAS; thereafter the levels of most parameters, especially soluble protein, Rubisco LSU and, at late stages of senescence, insoluble protein, were higher in pruned than in control shoots. Thus, shoot pruning slows down senescence. The 56- and 78-kDa polypeptides increased, rather than decreased, with leaf age; the level of these two polypeptides showed a negative relationship with Rubisco LSU (r = -0.933 and r = -0.758, respectively).     

Increased protein carbonylation in leaves of Arabidopsis and soybean in response to elevated [CO2]

While exposure of C3 plants to elevated [CO2] would be expected to reduce production of reactive oxygen species (ROS) in leaves because of reduced photorespiratory metabolism, results obtained in the present study suggest that exposure of plants to elevated [CO2] can result in increased oxidative stress. First, in Arabidopsis and soybean, leaf protein carbonylation, a marker of oxidative stress, was often increased when plants were exposed to elevated [CO2]. In soybean, increased carbonyl content was often associated with loss of leaf chlorophyll and reduced enhancement of leaf photosynthetic rate (Pn) by elevated [CO2]. Second, two-dimensional (2-DE) difference gel electrophoresis (DIGE) analysis of proteins extracted from leaves of soybean plants grown at elevated [CO2] or [O3] revealed that both treatments altered the abundance of a similar subset of proteins, consistent with the idea that both conditions may involve an oxidative stress. The 2-DE analysis of leaf proteins was facilitated by a novel and simple procedure to remove ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from soluble soybean leaf extracts. Collectively, these findings add a new dimension to our understanding of global change biology and raise the possibility that oxidative signals can be an unexpected component of plant response to elevated [CO2].     

Development of leaf photosynthetic parameters in Betula pendula Roth leaves: correlations with photosystem I density

The global modelling of photosynthesis is based on exact knowledge of the leaf photosynthetic machinery. The capacities of partial reactions of leaf photosynthesis develop at different rates, but it is not clear how the development of photoreactions and the Calvin cycle are co-ordinated. We investigated the development of foliar photosynthesis in the temperate deciduous tree Betula pendula Roth. using a unique integrated optical/gas exchange methodology that allows simultaneous estimation of photosystem I and II (PS I and PS II) densities per leaf area, interphotosystem electron transport activities, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. We combined these measurements with in vitro determinations of Rubisco, soluble protein and chlorophyll contents. We observed a strong increase in leaf photosynthetic capacity in developing leaves per leaf area, as well as per dry mass, that was paralleled by accumulation of leaf Rubisco. Enhanced mesophyll conductance was the outcome of increased carboxylation capacity and increased CO(2) diffusion conductance. However, Rubisco was only partly activated in the leaves, according to in vivo measurements of Rubisco kinetics. The amount of active Rubisco increased in proportion with development of PS I, probably through a direct link between Rubisco activase and PS I electron transport. Since the kinetics for post-illumination P700 re-reduction did not change, the synthesis of cytochrome b(6)f complex was also proportional to PS I. The synthesis of PS II began later and continued for several days after reaching the full PS I activity, but leaf chlorophyll was shared equally between the photosystems. Due to this, the antenna of PS II was very large and not optimally organized, leading to greater losses of excitation and lower quantum yields in young leaves. We conclude that co-ordinated development of leaf photosynthesis is regulated at the level of PS I with subordinated changes in PS II content and Rubisco activation.     

Proteolytic activities and ribulose 1,5 bisphosphate carboxlyase degradation in leaves of soybean (Glycine max L. Merril) with different nitrogen status

Changes in soluble proteins and Rubisco (E.C.4.1.1.39) contents were examined in leaves of nitrogen-deprived and nitrogen-sufficient soybeans. Rubisco content was very responsive to nitrogen stress, and this protein appeared to be the largest source of mobilizable nitrogen in the senescent leaf. Loss of soluble proteins and Rubisco was associated with a decrease in the activities of several proteolytic enzymes measured using artificial substrates: carboxypeptidase, aminopeptidase and haemoglobinase.The in vitro activity of enzyme(s) which can degrade Rubisco was investigated using endogenous Rubisco and in vitro radiolabelled Rubisco as substrates. Highest endopeptidic cleavage of endogenous Rubisco occured at pH 4; the enzyme responsible for this breakdown appeared to be a sulfhydryl-dependent proteinase. In contrast, [¹⁴C] Rubisco was attacked preferentially at pH 9, by a peptide hydrolase sensitive to EDTA. No increase in Rubisco-degrading activities was detected in nitrogen-deficient soybean leaves compared to control plant leaves.Abbreviations EDTA Ethylenediaminetetraacetate - LS Large Subunit of Rubisco - NEM N-ethylmaleimide - pCMB Parachloromercuribenzoate - PMSF Phenylmethylsulfonyl-fluoride - Rubisco Ribulose 1,5 Bisphosphate Carboxylase/Oxygenase     

Senescence in wheat leaves: is a cysteine endopeptidase involved in the degradation of the large subunit of Rubisco?

In wheat (Triticum aestivum L.), leaf senescence can be initiated by different factors. Depending on the plant system (intact plants or detached leaves) or the environmental conditions (light, nutrient availability), the symptoms of senescence differ. The aim of this work was to elucidate the catabolism of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC. 4.1.1.39) under various senescence-inducing conditions. Leaf senescence was initiated in intact plants by darkness or by N-deprivation and in leaf segments by exposure to light or darkness. Depending on the treatment, a 50 kDa fragment of Rubisco was observed. The formation of this fragment was enhanced by leaf detachment and low light. In segments exposed to high light and in intact plants induced to senesce by N-deprivation, the fragment was essentially absent. Since an antibody against the N-terminus of a large subunit of Rubisco (LSU) did not cross-react with the fragment, it appears likely that a smaller fragment was removed from the N-terminus of LSU. Inhibitor studies suggest that a cysteine endopeptidase was involved in the formation of the 50 kDa fragment. Non-denaturing-PAGE followed by SDS-PAGE revealed that the fragment was produced while LSU was integrated in the holoenzyme complex, and that it remained there after being produced. It remains open how the putative endopeptidase reaches the stromal protein Rubisco. The results indicate that depending on the senescence-inducing conditions, different proteolytic enzymes may be involved. The involvement of vacuolar proteases must be considered as occurring during LSU degradation, which takes place in darkness, low light or under carbon limitation.     

Sugars have potential to curtail oxygenase activity of Rubisco

Sugars play a critical role in regulating overall cellular metabolism and owing to their general compatibility with various cellular events plants invariably show enhanced levels of sugars for maintaining desired osmoticum under osmotic stress. Sugars (sucrose and trehalose) and sugar-alcohols (glycerol, mannitol, inositol, and sorbitol) with the exception of sorbitol lowered oxygenase activity of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase, EC 4.1.1.39) without altering carboxylase activity under unstressed conditions. Most interestingly, these solutes including sorbitol fully curtailed NaCl-induced enhancement in oxygenase activity, even at concentrations as low as 50mM. However, none of these solutes could alleviate NaCl-suppressed carboxylase activity. In summary, our findings demonstrate that one of the most important roles of sugars and sugar-alcohols in plants exposed to salt stress is to curtail oxygenase activity of Rubisco.     

Degradation of the Large Subunit of Ribulose-1, 5-Bisphosphate Carboxylase／Oxygenase in Wheat Leaves

The degradation of the large subunit (LSU) of ribulose- 1, 5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) in wheat (Triticum aestivum L. cv. Yangmai 158) leaves was investigated. A 50 kDa fragment, a portion of the LSU of Rubisco, was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with antibody against tobacco Rubisco in crude enzyme extract of young wheat leaves. The appearance of the 50 kDa fragment was most obvious at 30-35 ℃ and pH 5.5. The LSU and its 50 kDa fragment both existed when the crude enzyme extract was incubated for 60 min. The amount of LSU decreased with incubation time from 0 to 3 h in crude enzyme extract. However, the 50 kDa fragment could not be found any pH from 4.5 to 8.5 in chloroplast lysates of young wheat leaves. In addition,through treatment with various inhibitors, reactions were inhibited by cysteine proteinase inhibitor E-64 or leupeptin.